Method for producing ammoniac from methanol

ABSTRACT

Ammonia is catalytically produced from a nitrogen-hydrogen mixture. First of all, a vaporous feed mixture, which comprises 30 to 60 vol-% methanol and 40 to 70 vol-% steam and has a volume ratio steam:methanol of 1-3, is passed through at least one bed of a breakdown catalyst at pressures in the range from 30 to 200 bar, the temperatures in the catalyst bed lying in the range from 200 to 500° C. From the catalyst bed, a first gas mixture is withdrawn, which, calculated dry, comprises 40 to 80 vol-% H 2  and 10 to 30 vol-% CO 2 . The first gas mixture is cooled, CO 2  is removed in a gas cleaning, and a second gas mixture is generated, which comprises at least 95 vol-% nitrogen and hydrogen, and which is supplied as synthesis gas to an ammonia synthesis for the catalytic production of ammonia.

[0001] This invention relates to a process for the catalytic productionof ammonia from a nitrogen-hydrogen mixture.

[0002] From German patent 20 07 441 the generation of an ammoniasynthesis gas is known, where by gasifying hydrocarbons a raw gas isgenerated, which is desulfurized, converted, liberated from CO₂ andfinally subjected to washing with liquid nitrogen for removing residualimpurities. In EP patent 0307983 a similar process is described, whereinprior to the synthesis of ammonia converted synthesis gas is subjectedto washing with liquid nitrogen. Details of the catalytic production ofammonia can be found in Ullmann's Encyclopedia of Industrial Chemistry,5th edition, vol. A2, pp. 143-215; the production of urea is describedthere in vol. A27, pp. 333-350.

[0003] It is the object underlying the invention to be able to produceammonia on an industrial scale, even if no fossil fuels are available.In accordance with the invention this is achieved in that a vaporousfeed mixture, which comprises 30 to 60 vol-% methanol and 40 to 70 vol-%steam and has a volume ratio steam to methanol of 1-3, is passed throughat least one bed of a breakdown catalyst at pressures in the range from30 to 200 bar, the temperatures in the catalyst bed lying in the rangefrom 200 to 500° C., that a first gas mixture is withdrawn from thecatalyst bed, which, calculated dry, comprises 40 to 80 vol-% H₂ and 10to 30 vol-% CO₂, that the first gas mixture is cooled, CO₂ is removedfrom the first gas mixture in a gas cleaning, and a second gas mixtureis generated, which comprises at least 95 vol-% nitrogen and hydrogen,and which is supplied as synthesis gas to an ammonia synthesis for thecatalytic production of ammonia. In this process, the carbon-containingcomponents of the feed mixture should expediently comprise 80 to 100vol-% methanol.

[0004] The ammonia synthesis may comprise one or several stages; one ofthe possible variants consists in that in the ammonia synthesis thesynthesis gas is passed through at least two catalyst-containingreactors, where in one reactor the synthesis gas is used as coolingmedium for indirectly cooling the catalyst.

[0005] One embodiment of the invention consists in that the ammoniagenerated in the ammonia synthesis is at least partly converted to ureaby reaction with CO₂. Expediently, the CO₂ removed in the gas cleaningis at least partly recovered and used for producing urea.

[0006] The reaction of the feed mixture at the breakdown catalyst is anendothermal process, where chiefly the following reactions take place:

[0007] It may be expedient to pass the feed mixture through several bedsof a breakdown catalyst, at least one bed being indirectly heated bymeans of a heating fluid.

[0008] For the gas cleaning of the first gas mixture various methodsknown per se may be used and also be combined with each other. Onepossibility is to remove CO₂ in the gas cleaning by physically washingwith methanol at temperatures in the range from −20 to −70° C. Theremoval of CO₂ may, however, also be effected by pressure-swingabsorption; furthermore, pressure-swing absorption for the removal ofCO₂ may also be performed before physically washing with methanol.

[0009] When CO₂ has been removed from the first gas mixture, a finecleaning is recommended. This fine cleaning may for instance be effectedby washing with liquid nitrogen, and a second gas mixture is withdrawntherefrom, which substantially consists of H₂ and N₂ and can be suppliedto the ammonia synthesis as synthesis gas. Upon removal of CO₂, thefirst gas mixture is alternatively passed through a catalyticmethanization, in which CO is reacted with H₂ to obtain CH₄. ResidualCO₂ is also reacted to obtain methane.

[0010] Embodiments of the process will be explained with reference tothe drawing. The drawing shows a flow diagram of the process.

[0011] Methanol, which usually has a pressure of 30 to 200 bar andpreferably at least 100 bar, is supplied via line (1) and mixed withwater from line (2). Via line (1 a), the mixture is supplied to a firedheater (3), and in line (4) there is obtained a vaporous feed mixturewith temperatures in the range from 200 to 500° C. and preferably atleast 300° C., which is charged into a gasification reactor (5). Thereactor (5) includes two beds (5 a and 5 b) with breakdown catalysts,which in the upper bed (5 a) may be a Zn—Cr catalyst and in the lowerbed (5 b) a Cu—Zn breakdown catalyst. The mixture coming from the upperbed (5 a) enters the lower bed (5 b) usually with temperatures of 200 to350° C. and preferably at least 250° C. In the present case, a heating(6) by indirect heat exchange by means of a heating fluid is provided inthe lower bed.

[0012] The first gas mixture leaving the gasification reactor (5) vialine (7) chiefly comprises H₂ and CO₂, it furthermore contains CO, CH₄and methanol as well as possibly small amounts of further impurities.This gas mixture is first of all cooled in a heat exchanger (8) and thensupplied through line (9) to a washing column (10), in which it iscooled in a countercurrent flow with water sprayed in from line (11).What condenses above all is methanol, which together with the usedwashing water is discharged via line (2).

[0013] The washed gas is then supplied through line (13) to a gaswashing plant (14), to in particular remove CO₂. This may be effectedfor instance by physically washing with methanol at temperatures in therange from about −70 to −20° C.; another possibility is e.g. washing bymeans of methyldiethylamine or selexol washing. Used, CO₂-containingwashing solution is withdrawn via line (16) and charged into aregeneration (17), in order to remove CO₂ from the washing solution.Regenerated washing solution is recirculated to the gas washing plant(14) via line (18). The CO₂ obtained is very well suited for beingsupplied through line (20) to a urea synthesis (21). The amount of CO₂obtained usually is sufficient for converting the entire ammoniaproduced into urea.

[0014] Partly cleaned synthesis gas is withdrawn from the gas washingplant (14) via line (22) and is treated in a second washing plant (23),where liquid nitrogen is used as washing liquid. The required nitrogencomes e.g. from an air-separation plant and is supplied via line (26).Details of washing with liquid nitrogen for generating an NH₃ synthesisgas can be found in EP patent 0307983, which has already been mentionedabove. In the washing plant (23), there is usually obtained an exhaustgas containing CO and CH₄, which through line (42) is supplied as fuelto the heater (3).

[0015] Washing (23) is performed such that the synthesis gas obtained inline (24) already has a molar ratio H₂: N₂ of about 3:1. This synthesisgas is heated in the indirect heat exchanger (45) and flows through line(24 a) to an ammonia synthesis, to which belong the indirectly cooledreactor (25) and the adiabatically operated reactor (26). Circulatingsynthesis gas from line (27) together with fresh synthesis gas of line(24 a) enters the reactor (25) through line (27 a) with temperatures inthe range from 100 to 200° C., in which reactor it flows through tubes(28) or passages, the gas serving as cooling medium and dissipating heatfrom the catalyst bed (25 a).

[0016] The synthesis gas leaves the reactor (25) via line (29) withtemperatures in the range from 300 to 500° C., and in the reactor (26)it gets in contact with the catalyst thereof, which forms a bed. TheNH₃-forming reaction is an exothermal reaction, so that the mixtureflowing off via line (30) has temperatures of 400 to 600° C. and ispassed through a cooler (31). Subsequently, the NH₃-containing synthesisgas coming from line (32) enters the reactor (25) and flows through theindirectly cooled catalyst bed thereof. The outlet temperature in line(33) lies in the range from 300 to 500° C. and preferably 380 to 430° C.The product mixture in line (33) has an NH₃ concentration of at least 20vol-%; in addition, it chiefly contains N₂ and H₂. This mixture issubjected to a multi-stage cooling (34) and finally reaches a separator(35), from which NH₃ is withdrawn through line (36) in liquid form. Thegaseous components are withdrawn via line (27) and are recirculated asrecycle gas.

[0017] The NH₃ produced can wholly or partly be removed through line(37) and be supplied to a use known per se. Furthermore, the NH₃ maywholly or partly be supplied through line (38) to a urea synthesis,which is known per se. Urea produced is withdrawn via line (39).

EXAMPLE

[0018] In a procedure corresponding to the drawing, 42 t methanol areprocessed per hour to obtain 41.7 t ammonia. The subsequent tableindicates the flow rates (t/h), the gas compositions (vol-%) as well astemperature and pressure in various lines. The data are calculated inpart. Line 4 9 13 24a 27a 33 Flow rate 138 138 63 42 127 127 Temp. (°C.) 350 250 30 32 175 403 Pressure 149 148 146 143 143 140 (bar)Composition: CO₂ 0.8 14.9 24.4 — — — CO — 0.4 0.7 — — — N₂ — — — 25.224.4 19.0 H₂O 66.2 32.2 0.1 — — — NH₃ — — — — 4.8 27.0 CH₃OH 33.0 8.5 —— — — H₂ — 44.0 74.8 74.8 70.8 54.0

[0019] The catalyst of the upper bed (5 a) chiefly consists of Zn andCr, and the catalyst of the lower bed (5 b) chiefly consists of Cu andZn. The catalysts of the ammonia synthesis are commercially available(manufacturer e.g. Süd-Chemie, München (Germany), Type G-90 and AS-4).The gas washing (14) employs methanol as washing liquid, and thetemperature in the washing zone is about −58° C. Fine cleaning iseffected by washing with liquid nitrogen in accordance with EP patent0307983. In line (20), 54 t/h CO₂ are obtained, which is virtually pureand can be used for a urea synthesis.

1. A process for the catalytic production of ammonia from anitrogen-hydrogen mixture, characterized in that a vaporous feedmixture, which comprises 30 to 60 vol-% methanol and 40 to 70 vol-%steam and has a volume ratio steam methanol of 1-3, is passed through atleast one bed of a breakdown catalyst at pressures in the range from 30to 200 bar, the temperatures in the catalyst bed lying in the range from200 to 500° C., that a first gas mixture is withdrawn from the catalystbed, which, calculated dry, comprises 40 to 80 vol-% H₂ and 10 to 30vol-% CO₂, that the first gas mixture is cooled, CO₂ is removed from thefirst gas mixture in a gas cleaning, and a second gas mixture isgenerated, which comprises at least 95 vol-% nitrogen and hydrogen andwhich is supplied as synthesis gas to an ammonia synthesis for thecatalytic production of ammonia.
 2. The process as claimed in claim 1,characterized in that the ammonia generated in the ammonia synthesis isat least partly converted to urea by reaction with CO₂.
 3. The processas claimed in claim 1 or 2, characterized in that the CO₂ removed in thegas cleaning is at least partly recovered and used for producing urea.4. The process as claimed in claim 1 or any of the preceding claims,characterized in that the feed mixture is passed through several beds ofa breakdown catalyst, at least one bed being indirectly heated by meansof a heating fluid.
 5. The process as claimed in claim 1 or any of thepreceding claims, characterized in that upon removal of CO₂ the firstgas mixture is passed through a washing with liquid nitrogen and thesecond gas mixture is withdrawn therefrom, which upon heating issupplied as synthesis gas to the ammonia synthesis.
 6. The process asclaimed in any of claims 1 to 4, characterized in that upon removal ofCO₂ the first gas mixture is passed through a methanization, in which COis reacted with H₂ to obtain CH₄.
 7. The process as claimed in claim 1or any of the preceding claims, characterized in that in the gascleaning CO₂ is removed in a physical washing with methanol attemperatures in the range from −20 to −70° C.
 8. The process as claimedin any of claims 1 to 4, characterized in that in the gas cleaning CO₂is removed by pressure-swing adsorption.
 9. The process as claimed inclaim 1 or any of the preceding claims, characterized in that in theammonia synthesis the synthesis gas is passed through at least twocatalyst-containing reactors, where in one reactor the synthesis gas isused as cooling medium for indirectly cooling the catalyst.